Generally, there are two methods for cutting a steel plate into various shapes: one in which cutting is started from an end of the steel plate, and one in which cutting is started from a desirable position in the steel plate surface. The latter is a piercing start method in which a through hole is formed in the steel plate at a predetermined position, and in which cutting is started from this through hole (which operation will hereinafter be referred to as "piercing start"). This piercing start, in contrast with the former, enables any desired shape in the steel plate surface as well as a round or triangular shape to be cut. Plasma cutting methods are also based on the above two methods. In such cutting method, with respect to piercing start in particular, a plasma cutting apparatus such as the one described below is employed and there are serious problems relating to it. That is, as shown in FIG. 6a, before a hole 311 is formed through a member 30 to be cut (which operation is hereinafter referred to as "piercing"), the cut material melted by a plasma torch 12 is blown to a portion in the vicinity of the hole 311, attaches thereto, and is further accumulated as slag 32. As shown in FIG. 6b, the slag 32 interferes with the extreme end of the plasma torch 12 and acts as a cause of damage thereto when the plasma torch 12 is moved. Further, in the state shown in FIG. 6a, the melt which is blown up attaches to the extreme end of the plasma torch 12 so that the plasma jetting is obstructed, and therefore the melt may cause a reduction in cutting quality as well as damage to the plasma torch. The problem of such a drawback is very serious with respect to cutting of a thick plate by plasma cutting, because the amount of molten material is increased if the thickness of the member to be cut is increased. Conventionally, some methods are used to cope with this problem, as described below. At the time of piercing, the height of the plasma torch is increased and piercing is performed in this state, and the height of the plasma torch is thereafter reduced and cutting is performed, or cutting is started from a high position, thereby avoiding occurrence of the troublesome condition. Such methods, however, are not suitable as a means for solving the problem if improvements in quality with respect to piercing accuracy and cutting accuracy are required.
A slag-deposition preventive is known. Conventionally, a slag-deposition preventive is used in such a manner that it is applied to portions of a member to be cut including a cut end of the reverse surface and the vicinity of the same during cutting. It is used for the purpose of improving the cutting quality (e.g., appearance) of the reverse surface of the cut member by preventing deposition of slag on the portion to which the preventive is applied. That is, no idea is known which connects with a method of applying a slag-deposition preventive to the portion to be pierced to obtain desirable effects.
In view of the above-described problems of the prior art, an object of the present invention is to provide a plasma cutting method which enables a suitable piercing start and to provide a plasma cutting apparatus suitable for practicing this method.